The present invention relates to a fabrication method of fine structures to be used in quantum effect devices or the like.
When the structure of a material becomes finer than the phase coherent length of electrons in the material, more specifically, than the magnitude of about 100 nanometers (nm) to several micrometers (um) in the case of a semiconductor, the behavior of the electrons in the material will start to show electron wave inteference effects that cannot be observed with the electrons in a bulk material.
In other words, as the structure of the material becomes finer than the de Broglie wave length (several tens of nm), various accompanying effects (such as the tunnel effect, the effect due to quantum level formation, the effect due to mini-zone formation or the like) will be showing up with the electrons of the material. These effects are generally referred to as the quantum effects.
In recent years, studies have been made to realize devices by making use of these effects.
As a result, use of a film formation technique of atomic scale such as the molecule beam epitaxial growth method or the like has so far resulted in creation of a good semiconductor superlattice structure and a modulation dope structure, leading to realization of high electron mobility transistor (HEMT), heterojunction bipolar transistor (HBT), multi-quantum well (MQW) lasers or the like.
All of these are made from fine structures wherein freedom of electrons in the stacking direction of the semiconductor layer is restricted.
On the other hand, various studies have been conducted to realize super high speed transistors, super low threshold lasers or the like. In these studies, such effects as a scatter vanishing effect of electrons, an effect reflecting the electron's discrete density of states or the like (e.g., the effect due to so called two-dimensional electron systems and three dimensional electron systems) are utilized through creation of a quantum wire or a quantum box to enhance the dimension of the restriction imposed on the freedom of electrons to a two-dimensional or three-dimensional level.
There have been so far devised a variety of methods of making a quantum wire or a quantum box including: (1) a method to form a potential barrier by either removing or making mixed crystal of a portion of the superlattice structure by means of focused ion beam or the like after formation of a superlattice structure, (2) a method of half atom alternate epitaxy applied on an off-orientation substrate, and (3) a method making use of crystal orientation dependence in crystal growth speed as observed in facet growth and in crystal growth inside channels formed on a substrate.
However, prior art method (1) to form a potential barrier (by either removing or making mixed crystal of a portion of the superlattice structure by means of focused ion beam or the like after formation of the superlattice structure) had the problems that the interface steepness was not sufficient due to the mutual diffusion of the constituting atoms. The carrier trap center density in the interface became large on account of lattice defects or the like caused during processing. Particularly, with an optical device or the like wherein the reversed layer of the interface cannot be used, as in the case of an electron device using single carriers, a good quantum structure was not able to be formed.
On the other hand, the prior art methods of (2) (using half atom alternate epitaxy on the off-orientation substrate) and (3) (making use of methods such as facet growth and crystal growth inside the channels or the like) showed good crystallization in the interface of fine structure when compared with (1). But the fact that structures of arbitrary configurations could not be obtained due to the restrictions imposed by the crystalline direction of the substrate was a problem.